A liquid crystal display device has become rapidly widespread recently in place of a cathode ray tube (CRT) display device. Such a liquid crystal display device is for widely use in an electronic device such as a liquid crystal display television, a monitor, or a mobile phone, because the liquid crystal display device has the advantages that it is energy-saving, thin, and light. It is possible to further put such advantages to good use by, for example, improving an illumination device (a so-called backlight), which is provided behind the liquid crystal display device.
The illumination device is broadly classified into a side illumination device (also called “an edge illumination device”) and a direct illumination device. The side illumination device is arranged such that (i) a light guide plate is provided behind a liquid crystal display panel and (ii) a light source is provided on a lateral end of the light guide plate. Such a side illumination device uniformly irradiates the liquid crystal display panel as follows. The light emitted from the light source is reflected by the light guide plate, and is then directed toward the liquid crystal display panel. According to the arrangement, it is possible to achieve a thinner illumination device that is excellent in uniformity of luminance, although such an illumination device is not so excellent in luminance level. Because of its excellent uniformity of luminance, the side illumination device is mainly employed in a medium-small size liquid crystal display for use in a device such as a mobile phone or a laptop computer.
The direct illumination device is such that a plurality of light sources are provided behind the liquid crystal panel so as to directly irradiate the liquid crystal panel. The direct illumination device thus easily achieves high luminance even in a case where it is used in a large display. Therefore, the direct illumination device is mainly employed in a liquid crystal display that is as large as 20 inches or more. However, a conventional direct illumination device is some approximately 20 mm to 40 mm in thickness, which is a problem to be solved for further reducing a thickness of the display.
In order to further reduce a thickness of a large liquid crystal display, the light sources and the liquid crystal display panel should be provided closer to each other. In doing so, the number of light sources needs to be increased so as to achieve uniformity of luminance of the illumination device. However, the increase in the number of the light sources causes cost increase. Under such circumstances, it is desired to develop, without increasing the number of the light sources, an illumination device that is thin and excellent in uniformity of luminance.
In order to solve these problems, an attempt has been conventionally carried out to reduce, by employing an illumination device constituted by arranging a plurality of light guide units, a thickness of a large liquid crystal display device.
For example, Patent Literature 1 discloses a technique of achieving a thin illumination device by arranging light guide units so as to overlap one another. This is described below with reference to FIG. 10.
FIG. 10 is a partial cross-sectional view illustrating the illumination device of Patent Literature 1. As illustrated in FIG. 10, an illumination device 180 includes: a plurality of light guide units; and a diffusing sheet 175. Each of the plurality of light guide units includes (i) a fluorescent tube 171, (ii) a light guide 172 having an incidence surface 172a, an emitting surface 172b, and a reflection surface 173 that reflects light toward the emitting surface 172b, and (iii) a reflection mirror 174.
The plurality of light guide units overlap each other such that (i) they are at an angle and (ii) a side surface of each of the light guides 172 is adjacent to another reflection surface 173 of a neighboring light guide 172. The emitting surface 172b of each of the light guides 172 serves as a light emitting surface, and is at an angle θ of 30 degrees to the reflection surface 173.
Patent Literature 1 teaches that, according to the configuration, it is possible to achieve a thin illumination device 180 that has high use efficiency of light beams and thus has high luminance.